I’ve done a fair bit of AR builds and mods over the years culminating with a flurry of them this summer. I’ve used an old plastic clam shell upper receiver block, and for the most part it’s been fine. I’ve occasionally run into an upper profile that doesn’t fit the block though, and more recently had a couple barrels that must have been torqued on by gorillas. I could see the aluminum upper flexing in the vise and backed off before I damaged something.
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